LibreELEC aside, did it work right when running Kodi through Android on that box?
First, try bypassing the receiver and connect directly to your projector. Make sure you're using HDCP 2.2 / HDMI 2.0 cables, and test more than one if it doesn't work. Worth noting is that the HDMI cables that come with these Android boxes are notoriously cheap and horrible, so use something else.
If any of that works, try connecting your box to HDMI input 4 / HDCP 2.2 input on your AV receiver. I've noticed that the Nexbox a95x doesn't play nice with my brother's 4K TV unless it's plugged into one of the UHD inputs. Different box, but same chipset... so it might be having similar problems. If that still doesn't work, then try them all.
Usually it would be that either the projector or AV receiver may not support the refresh rate of the video you're trying to play, so after attempting to switch, Kodi will default to the best available match if it doesn't get what it's looking for. Not sure what refresh rate your content is, but I'll assume 24hz if you're attempting to play back content that's not 60hz and getting a switch attempt before it fails back over. The box itself supports it, and both your projector and your AV receiver should support it from what I read off the specs page... So I honestly don't know why it wouldn't work outside of potential cable or input mismatches. Alternating the HDMI input port and also making sure you have HDMI 2.0 compliant cables both to and from the AV receiver may help.
Well, I removed the disp_cap_hack and its working. while before without disp_cap_hack it didn't.
But its working now 
